1. Field of the Invention
The present invention pertains to ice detectors that can be used particularly in aircraft.
2. Description of the Prior Art
There are detectors such as this that, in fact, use simple optical level detectors of the kind existing in the walls of certain containers. The detector has an optical channel that connects a light source to a receiver. A part of the wall of the optical channel forms an interface with the external environment. For a container, the environment external to the optical channel is the interior of the container. For ice detectors, the external environment is the one in which the vehicle is moving. In these detectors, the light from the source reaches the interface and, depending on the refractive index of the external environment, it is either reflected towards the receiver or refracted, at least partially, towards the external environment. It is reflected when the external environment consists of air and it refracted when there is ice or a liquid. The receiver thus receives varying amounts of light, and a comparison circuit compares the output signal from the receiver with a reference signal and triggers an ice alarm signal or a signal that a level has been reached when the comparison shows that the output signal from the receiver is below the reference value, namely, that refraction has occurred. The alarm signal thus obtained is an unreliable indication for the detection of ice. For, it is practically impossible to distinguish between rain and ice in this way because the refractive indices of water in the liquid state and of ice are very close to each other, and experience shows that a thin film of rain has the same effect as a layer of ice, irrespectively of its thickness.
The present invention is aimed at preventing or, at least, at reducing the above-mentioned drawbacks.
This goal has been achieved after observation of the phenomenon of the flow of rainwater on a wall of a moving vehicle. When the speed is sufficient for the air to sweep the wall at the place where the observation is made, the rainwater does not flow uniformly but in successive droplets or wavelets. This causes a modulation of the light received by the receiver and, hence, of the signal delivered by the receiver. Now, such a phenomenon cannot take place with ice because, even when it is not deposited homogeneously, ice practically does not flow on the wall of the vehicle.